Heretofore, copper alloys with soft alloying additives such as Pb and Bi (referred to as “lining”) have been used for the plain bearing described above. Metallic Pb or Sn or their alloys have been used for the overlay. Cu of the copper alloy diffuses into the overlay and forms Cu—Sn based intermetallic compounds such as Cu6Sn5, Cu3Sn, and the like, which in turn deteriorate fatigue resistance and softness of the overlay.
Therefore, an intermediate metallic layer, such as an Ni intermediate layer referred to as an Ni barrier, is usually interposed between the lining and the overlay.
Patent Documents 1 through 12 relating to plain bearings are listed below in filing date sequence. Patent Document No. 13 relates not to a plain bearing but to an Sn alloy/Cu alloy lamination structure, but also serves as a prior art document.
Patent Document 1, Japanese Patent No. 3397334 (Japanese Patent Application No. Hei3-302668), relates to a plain bearing which comprises a plain-bearing layer consisting of kelmet or aluminum alloy and an Sn-based overlay, which is applied on the plain-bearing alloy-layer. An intermediate layer made of Ni or the like for preventing diffusion is optionally interposed between the plain-bearing layer and the Sn-based overlay. Such layer structure is usual in the plain bearing. This patent document describes that organic carbon contained in the overlay in an amount of 0.02 to 0.5% by weight prevents diffusion of Sn into the plain-bearing layer. As a result, seizure resistance is improved.
In Patent Document 2, Japanese Unexamined Patent Publication (kokai) No. Hei5-239696 (Japanese Patent Application No. Hei 4-239696), an Sn-based overlay with hard matter additives is bonded on an underlying layer with higher adhesive strength. Granular hard particles are dispersed in an upper layer of the proposed Sn-based overlay. The granular hard particles are dispersed in an amount of 5 to 50% by volume and have an average particle diameter of 0.1 to 5 μm, with the largest diameter being smaller than the thickness of an oil film. A lower layer consists of Sn (alloy) free of dispersed hard matter. Copper alloy of the plain bearing is bronze or lead bronze containing In, Tl, P, Ag, Ni, Al or the like as an optional element(s) in a total amount of 5% or less.
In a Cu matrix of a plain-bearing copper-alloy according to Patent Document 3: Japanese Patent No. 3560723 (Japanese Patent Application No. Hei 8-57874), Ag, Sn, Sb, In, Mn, Fe, Bi, Zn, Ni, and/or Cr (except for combination of only Ag and Sn) is dissolved. Particularly, Ag is super-saturation dissolved under non-equilibrium. A minority phase of these elements is essentially not formed. Sn content ranges from 1 to 10 wt %, and Ni content ranges from 5 to 50 wt %. In this patent document, since the specified solute element(s), particularly Ag, concentrates on the surface of copper alloy, a material having improved seizure resistance is formed on this surface. Therefore, Sn, which is usually contained in the copper alloy, is not indispensable. In addition, an Ni intermediate layer interposed between the overlay and plain bearing for preventing diffusion of Sn is also not indispensable. An overlay based on Sn, Pb, or resin is not indispensable but may be used.
A plain bearing of Patent document 4, Japanese Unexamined Patent Publication (kokai) No. 10-205539 (Japanese Patent Application No. 10-205339) consists of a backing metal, a bearing-alloy layer formed on the backing metal and consisting of copper alloy, and an Sn or Al-based overlay. An intermediate layer consisting of Cu—Zn alloy with 20-50% by weight of Zn is interposed between the bearing-alloy layer and the overlay, so as to improve seizure resistance and fatigue resistance.
A plain-bearing alloy proposed in Patent document 5: Japanese Patent No. 3657742 (Japanese Patent Application No. Hei9-152160) contains 1% or more of at least one of Ag and Sn as an essential component and contains one or more of Sb, In, Al, Mg, or Cd as an optional element(s). The essential and optional element(s) are dissolved as a solute(s). During sliding, a compound of hexagonal crystal is formed in the upper part of the copper alloy or on the surface of a plain bearing.
Therefore, an Ni intermediate layer between the overlay and plain bearing for preventing diffusion of Sn is not indispensable for preventing diffusion of Sn. An overlay based on Sn, Pb, or resin is not indispensable but may be used. When an Ni intermediate layer for preventing diffusion is omitted as proposed by this patent document, cost reduction is attained.
Patent Document 6: Japanese Domestic Publication (PCT) No. Hei 11-510859 (Japanese Patent application Hei 10-500079) relates to a production method of a sliding member. It discloses dispersion of particles of carbide or the like in an electrolytically deposited material, such as Sn—Cu—Ni. Since the particles suppress diffusion of Sn, the so called-Ni barrier is unnecessary, and cost reduction can be attained.
A plain-bearing layer of a plain bearing proposed in Patent Document 7: Japanese Unexamined Patent Publication (kokai) 2000-64085 (Japanese Patent Application No. 10-345952) consists of copper alloy containing 50 to 95% by weight Cu, or aluminum alloy containing 60 to 95% by weight Al. A diffusion-suppressing layer consisting of Ni or the like is provided. An overlay is a plating layer of a lead-free alloy containing 8-30% by weight Cu, 60-97% by weight Sn, and 0.5-10% by weight Co. The overlay is characterized in that it does not embrittle at up to relatively high temperatures.
An overlay of a plain bearing proposed in Patent Document 8: Japanese Unexamined Patent Publication (kokai) No. 2000-345258 (Japanese Patent Application No. 11-163247) is an Sn alloy containing 2 to 10% by weight Ag.
An overlay of a plain bearing proposed in Patent Document 9, Japanese Domestic Publication (PCT) No. 2002-530530 (Japanese Patent Application No. 2000-582622) is an electro-plating layer, in which hard particles and soft particles are dispersed. Hardness increases in a direction toward a plain-bearing layer.
An overlay of a plain bearing proposed in Patent Document 10: Japanese Unexamined Patent Publication (kokai) 2001-247995 (Japanese Patent Application No. 2000-397944) is an Sn alloy, in the matrix of which Sn—Cu particles are dispersed. Wear resistance is improved over that of a conventional Pb-based overlay.
An Sn-based overlay of a plain bearing proposed in Patent Document 11: Japanese Unexamined Patent Publication (kokai) No. 2002-310158 (Japanese Patent Application No. 2001-109853) consists of a lower layer 4 and an upper layer 5 having different Cu contents. The lower layer 4 contains a greater amount of Cu, as high as 5 to 20% by weight, and is 1 to 3 μm in thickness. Cu of the Sn-based overlay may diffuse to an intermediate layer for preventing diffusion. Diffusion initiates at the lower layer 4. Therefore, Cu in the upper layer 5, which is in contact with opposite material, decreases by a suppressed amount. Decrease of Cu in a surface part of the overlay layer can be prevented for a long period of time, even in the intermediate layer for preventing diffusion is not thick.
Patent Document 12: Japanese Unexamined Patent Publication (kokai) No. 2006-153193 (Japanese Patent Application No. 2004-34398) proposes subjecting a copper-alloy lining and an Sn-based overlay to heat treatment. A Cu—Sn compound layer having a large amount of minute surface unevenness is formed by heat treatment in the Sn-based overlay. Both compatibility and wear resistance of the Sn-based overlay are maintained at enhanced levels, when the Sn plating layer is embedded in the minute convexities.
Patent Document 13: Japanese Unexamined Patent Publication (kokai) No. 59-145553 (Japanese Patent Application No. 59-4655) relates to a copper-alloy structure used for brazing or the like. A copper-alloy bottom layer of the structure contains Ni in an amount of approximately 19% to approximately 28% as an essential element and contains Zn, Mn, Fe, Cr, Mg, P, and/or Mg as an optional element(s). The structure includes an Sn-based coating layer. The document discloses that Ni of the bottom layer suppresses formation of Cu—Sn based intermetallic compound between the bottom layer and the coating layer. As a result, brazing exposure life is improved.    Patent Document 1: Japanese Patent No. 3397334    Patent Document 2: Japanese Unexamined Patent Publication (kokai) Hei 5-239696    Patent Document 3: Japanese Patent No. 3560723    Patent Document 4: Japanese Unexamined Patent Publication (kokai) Hei 10-205539    Patent Document 5: Japanese Patent No. 3657742    Patent Document 6: Japanese Domestic Patent Publication (PCT) Hei 11-510850    Patent Document 7: Japanese Unexamined Patent Publication (kokai) 2000-64085    Patent Document 8: Japanese Unexamined Patent Publication (kokai) 2000-345258    Patent Document 9: Japanese Domestic Patent Publication (PCT) 2002-530530    Patent Document 10: Japanese Unexamined Patent Publication (kokai) 2001-247995    Patent Document 11: Japanese Unexamined Patent Publication (kokai) 2002-310158    Patent Document 12: Japanese Unexamined Patent Publication (kokai) 2006-153193    Patent Document 13: Japanese Unexamined Publication (kokai) Sho59-145553